1. Field of the Invention
The present invention relates to a liquid crystal display device, in particular, to a liquid crystal display device having a plurality of liquid crystal layers stacked to compose a display screen. In addition, the present invention relates to a liquid crystal display device capable of displaying an image with high quality and low power consumption.
2. Description of the Related Art
As display devices for use with OA electronic devices such as personal computers, word-processors, and EWS (Engineering Work-Stations), display devices for use with personal electronic devices such as electronic calculators, electronic books, electronic notebooks, and PDA, and display devices for use with portable electronic devices such as portable TV units, portable telephone units, and portable FAX units, flat display devices are becoming attractive. Since display devices for use with portable electronic devices are battery-driven, the power consumption should be reduced.
Conventionally, as flat type display devices, liquid crystal display devices (LCD), plasma display panels (PDP), flat CRTs, and so forth are known. Among these flat display devices, from a view point of low power consumption, the LCDs are the most suitable and have been widely used.
An LCD of which the user directly sees a picture on the screen of the display is referred to as direct view type. In addition, the direct view type LCD can be categorized as a transmission LCD and a reflective LCD. The transmission LCD has a light source such as a fluorescent lamp disposed on the rear side of liquid crystal cells. On the other hand, the reflective LCD uses peripheral light as its light source. Since the transmission LCD uses such a back-light, the reflective LCD is superior to the transmission LCD from a view point of the power consumption. This is because the back-light consumes a power of 1 W or higher. Thus, as displays for use with portable electronic devices such as portable digital assistance devices, reflective LCDs have been widely used.
In the reflective LCD, from a view point of efficiency of light, GH (Guest Host) type display mode is the most suitable for that does not use a polarizer. In the GH type color display, three layers of GH mode liquid crystal cells that respectively contain dichroic dye materials of three primary colors such as cyan, magenta, and yellow should be disposed.
To display color images in a wide color reproducing range with a reflective liquid crystal display device, the layered structure of the GH liquid crystal cells is the most suitable. On the other hand, in the structure of the RGB parallel arrangement or CMY parallel arrangement, with all pixels that compose a display screen, the same color cannot be shown at the same time. Thus, the color reproducing range of the LCD become narrow.
When a dot-matrix screen is formed of such three-layer GH liquid crystal cells, data signals corresponding to the picture information should be transmitted to respective pixels that compose the display screen. As the matrix driving method, simple matrix driving method and active matrix driving method are known.
Since the simple matrix driving method requires a sharp V-T (Voltage-Transmittance) characteristic, it is not suitable for driving GH liquid crystal of which the content of a liquid crystal material is small due to the presence of color agents.
As an active matrix type LCD, the structure of which MIM (Metal-Insulator-Metal) diodes or thin film transistors are used as active element (switching elements having a non-linear characteristics).
In the method employing MIMs, as the number of pixels that composes the display screen increases, the effective voltage applied to each pixel decreases. Thus, the effective voltage applied to each pixel may decrease to 5 V or less. Consequently, from a view point of the V-T characteristic of the currently available GH liquid crystal, the MIM method is not suitable for driving the GH liquid crystal.
On the other hand, in the TET method, a voltage applied to each pixel that composes the display screen can be freely set. Thus, the active matrix driving method using TFTs is suitable for the driving method of the GH liquid crystal.
From the above-described background, a reflective display device having the structure of a plurality of GH liquid crystal layers has been proposed in for example Japanese Patent Application No. 8-57531. Hereinafter, such a device is referred to as tri-layer GH LCD. In addition, a method for driving such a tri-layer GH LCD has been proposed in Japanese Patent Application No. 7-235357.
The above-described tri-layer GH LCD can be structured as a transmission LCD having a back-light instead of a reflective LCD having a reflecting plate. In this case, since a color filter is not required, a display device with high efficiency of light and low power consumption can be structured.
On the other hand, as a driving method that allows the power consumption of tri-layer GH color LCD to decrease, multi-field driving method (MF driving method) has been proposed. In the multi-field driving method, one frame picture is divided into a plurality of sub-fields that are sequentially displayed. Thus, the power consumption can be decreased. However, in the multi-field driving method, the picture quality may deteriorate due to line disturbance (Cf. Japanese Patent Application No. 6-248460; Go. Itoh et al. "Advanced Multi-Field Driving Method for Low Power TFT-LCDs", J. ITE Japan, Vol. 50, No. 5, pp. 563-569 (1996); Go. Itoh et al. "Improvement of Image Quality on Low Power TFT-LCDs using Multi-Field Driving Method", Euro Display '96).
This is because the user recognizes the variation of colors two-dimensionally (as a plane) and the variation of brightness one-dimensionally (as a line). Thus, when the LCD is driven by the multi-field driving method, the user recognizes the boundary between sub-fields displayed in different timings.
Thus, technologies that accomplish an LCD that satisfies high picture quality and low power consumption are required.